The present invention relates to a dry-type developer and a wet-type developer for use in electrophotography, which are capable of yielding improved image density, resolution, and image fixing property, without a strike-through phenomenon that developed images on one side of the copy sheet are visible from the other side (hereinafter this phenomenon is referred to as the strike-through phenomenon).
As a developer for use in electrophotography, a toner comprising polyethylene wax as a binder agent and an organic or inorganic pigment, such as carbon black and phthalo-cyanine blue, which is dispersed in the binder agent, is conventionally known. The toner particles of such a toner containing polyethylene wax have the advantage over other toner particles that a higher image density can be obtained. However, the toner particles have the shortcoming that the images are depressed and defaced in the course of the image transfer process in a copying machine, which degrades the image quality, in particular, image resolution.
A trial has been carried out to prevent the decrease of the sharpness of image and resolution by using an oxidized polyethylene wax instead of polyethylene wax as proposed in Japanese Laid-Open Patent Application No. 54-97157. However, when oxidized polyethylene wax is employed, there is a problem that high image density cannot be increased any further.
In a wet-type toner, low-molecular weight polyolefins having a molecular weight of 3,000 or less are in general use. However, the melting viscosity of such low-molecular weight polyolefins is so low that the previously mentioned strike-through phenomenon occurs when the heat (150.degree. C. to 250.degree. C.) is applied in the course of the image development, transfer and fixing process, so that the above polyolefins are totally unsuitable for duplex copying. Furthermore the image fixing property thereof is so poor that developed images can be readily erased by an eraser.
In addition to the above-mentioned polyethylene wax and oxidized polyethylene wax, polyolefins, polystyrene resin, acrylic resin, epoxy resin, polyester resin, polyolefin resin, and rubber are employed as binder agents for the conventional toners. However these resins also have the same or similar shortcomings as mentioned above more or less.